Fuel injectors with electromagnetic pin actuation are commercially available, and they differ greatly in how they combine good performance and modest cost. An injector with electromagnetic pin actuation is provided with an valve injection having a valve seat, which ends in an injection nozzle and is coupled with a pin capable of being displaced from a position where the valve seat is closed to a position where the valve seat is open by a thrust by an electromagnetic actuator and against the action of a spring capable of holding the pin in the closed position; in particular, the actuator comprises an electromagnet capable of displacing the pin from the closed position to the open position against the action of the spring.
Injectors with electromagnetic pin actuation work very well with low to medium fuel pressures, while critical situations can arise with high fuel pressures since the electromagnet may not be able to produce sufficient force to open the injector in short periods of time; for this reason, injectors with hydraulic pin actuation have been proposed, i.e. injectors in which the displacement of the pin from the closed position to the open position against the action of the spring happens through the effect of hydraulic forces.
An example of an injector with hydraulic pin actuation is provided by patent application EP-1036932-A2 or patent application EP-0921302-A2, in which a lower portion of the pin is housed in an injection chamber, which is delimited below by the valve seat of the injection valve, and an upper portion of the pin is housed in a control chamber, which houses the spring that keeps the pin in the closed position; fuel is fed constantly at pressure either to the injection chamber, which it leaves through the injection nozzle when the pin is in the open position, or to the control chamber. The control chamber is coupled to a control valve, which is actuated by an electromagnetic actuator so as to be displaced against the action of a control spring between a closed position and an open position, in which it puts the control chamber in communication with a low-pressure drainage environment. In use, when the control valve is closed, the pressure of the fuel in the control chamber is equal to the pressure of the fuel in the injection chamber, and the pin is held in the closed position either by the action of the spring or by the hydraulic force that is generated when the area of the pin subject to the action of the fuel is greater in the upper portion housed in the control chamber than in the lower portion housed in the injection chamber. When the control valve is open, the pressure of the fuel in the control chamber falls to much lower values than the pressure of the fuel in the injection chamber and the pin is displaced upwards into the open position by the effect of the hydraulic force that is generated by the difference in pressure.
Another example of an injector with hydraulic pin actuation is provided by patent application WO-0129395-A1, in which an upper portion of the pin is housed in the control chamber, while a lower portion of the pin is housed in an injection chamber, which is delimited below by the valve seat of the injection valve and houses the spring that holds the pin in the closed position; the control chamber is coupled to the control valve, which is actuated by a piezoelectric actuator so as to be displaced between a closed position, and an open position, in which it puts the control chamber in communication with a low-pressure drainage environment.
Patents U.S. Pat. No. 5,664,545-A1, DE-1016484-A, EP-0851115-A1 and EP-0999360-A1 supply further examples of injectors with hydraulic pin actuation.
The operation of the control spring is to hold the valve body of the control valve in the closed position with a predetermined elastic force that must be greater than the hydraulic force exerted by the fuel; clearly, the greater the working pressure of the fuel, the greater the elastic force that has to be exerted by the spring. As the working pressure of the fuel has gradually risen, higher-performance control springs are being used, capable of exerting ever-higher elastic forces; obviously, an increase in the elastic force exerted by the control spring that holds the valve body of the control valve in the closed position involves a corresponding increase in the force that has to be generated by the electromagnetic actuator of the control valve in order to move the control valve from the closed position to the open position. However, in known injectors with hydraulic pin actuation the increase in the force generated by the electromagnetic actuator of the control valve has proved problematic and has only been resolved by increasing the transverse dimension of the injectors.
As described by patent application IT-BO2002A000497, in order to obtain an increase in the force generated by the electromagnetic actuator of the control valve without a corresponding increase in the transverse dimension of the injector, a proposal has been made to use an electromagnetic actuator provided with a pair of electromagnets electrically independent of each other and provided with two respective moveable armatures, which are both mechanically connected to the valve body of the control valve. The electromagnetic actuator of the injector with hydraulic pin actuation described in patent application IT-BO2002A000497 is capable of producing a very great force while having a modest transverse dimension; however, such an actuator has proved relatively costly, complicated to assemble and complicated to develop.
GB2341893 relates to a two-stage electromagnetically actuated fuel injector for use in a common rail system of a i.c. engine. The fuel injector comprises a valve needle slidable in a bore and having an upper end exposed to pressure in a control chamber; the pressure in the control chamber is relieved by a valve to initiate injection. The valve member is movable by a first electromagnetic actuator which comprises a first component coupled to the valve member and a second component which is movable by a second electromagnetic actuator; thus injection can be made in two stages by energizing the actuator windings of the two actuators respectively. Alternatively, the valve member may be coupled to an armature movable by a single electromagnetic actuator having a winding located between relatively movable stator components defining respective pole faces which are spaced from the armature by different distances.